1. Field of the Invention
The present invention relates to avionic racking, and more particularly to an annular gasket utilized in mounting metering plates and sealing the electronic instruments on shelves.
2. Description of the Prior Art
In an aircraft, each piece of electronic equipment is supported on an individual tray which, in turn, is supported at its front and back ends on a pair of cross beams. The trays are made in standard ATR widths to accommodate the various sizes of instruments. In a typical shelf, the plurality of trays are closely spaced on the cross beams to completely enclose the entire length of the shelf. A closure plate is positioned beneath the cross beams to completely enclose the area directly below the trays to form an air plenum chamber. The air plenum chamber of each shelf then communicates with a vertical manifold, located at the ends of the shelves, which in turn is connected to a vacuum system.
The outer casing of each instrument is provided with standard air vents at the top and bottom sides to permit cooling air to be drawn therethrough. Each tray includes a pocket formed below the main surface thereof, with the pocket having a plurality of metering ports defined therein to enable the supported instrument to communicate with the air plenum chamber. Finally, an annular gasket is bonded to the bottom surface of the pocket of each tray about the periphery of the aperture with the upper portion of the gasket extending out of the pocket above the main surface of the tray to sealingly engage the bottom of the instrument supported thereon. In this manner, air from beneath the instrument is prevented from leaking into the air plenum chamber. As a result, the only cooling air drawn into the system is vented through the instruments.
Although these trays have been used extensively in a great number of aircraft, they still suffer from various shortcomings which severely hamper their utilization.
A serious problem is encountered when the instrument is installed on the tray. During such installation, the back end of the instrument is placed on the tray and shoved rearwardly until it contacts the backplate on the tray. However, during such movement, the bottom edge of the instrument contacts the exposed portion of the gasket extending above the surface of the tray. Quite often, this engagement causes the gasket to be torn away from its bonded seat on the tray. When this occurs, the gasket becomes twisted and entangled within the pocket. This unseating of the gasket not only causes leakage into the air plenum chamber from the area beneath the supported instrument, but the entanglement also causes misalignment of the supported instrument with the tray backplate and the electrical connections mounted thereon.
Another shortcoming with the prior trays is that if it is desired to change the porting arrangement in the tray because of different cooling requirements, the entire tray has to be replaced with one having the desired porting arrangement. Not only is this inconvenient because an extra supply of trays which must be kept by the user, but the storing of extra trays also involved an even greater expense.